1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. More particularly, the present invention relates to a semiconductor device having a high dielectric constant insulating film in a gate section. The invention also pertains to a method for manufacturing the semiconductor device.
2. Description of the Related Art
A Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) is known as one of elements used in a semiconductor integrated circuit device. FIGS. 10A and 10B are schematic views of a MOSFET. In the MOSFET, sizes in the longitudinal and lateral directions thereof are simultaneously reduced as shown in the transition from FIG. 10A to FIG. 10B in accordance with a scaling law, as well as the performance is improved while maintaining normal characteristics of the element.
However, accompanying miniaturization of the MOSFET, the following problem occurs. That is, as shown in the transition from a gate insulating film 21 (FIG. 10A) to a gate insulating film 21a (FIG. 10B), when a thickness of a gate insulating film is reduced to about 2 nm equivalent to several atomic layers, a current flows through the gate insulating film, that is, a tunneling current It begins to flow. As a result, a leakage current increases uncontrollably. Further, the leakage of a drain current as well as increase in power consumption occurs (see, e.g., Japanese Unexamined Patent Publication No. 2006-019615).
Particularly, the next-generation MOSFET is required to have the following characteristics. That is, even if further miniaturization is promoted, the threshold can be controlled as well as a leakage current, Equivalent Oxide Thickness (EOT: a value obtained by reducing a physical thickness of a material used for a gate insulating film to a thickness of a silicon oxide (SiO2) film indicating a dielectric capacity equivalent to that indicated by the material) and hysteresis are reduced.
In order to realize the above-described next-generation MOSFET, an application of a material (hereinafter, referred to as a high dielectric constant insulating film) with a dielectric constant higher than that of a conventionally used oxide film is proposed for a gate insulating film. Herein, there is studied a method for improving a dielectric constant by adding nitrogen (N) into an oxide film to increase a nitrogen concentration in an obtained oxynitride film.
However, the oxynitride film has the following problem. That is, when the nitrogen concentration in the oxynitride film is increased, an insulation property increases whereas an electric charge due to defects in the film increases to cause significant deterioration in an operation speed and reliability of the MOSFET. Therefore, a high dielectric constant material such as a hafnium oxide (HfO2) or aluminum oxide (Al2O3) newly used as an alternate material of the oxynitride film is proposed for a high dielectric constant insulating film.
By employing such a high dielectric constant insulating film, a Capacitance Equivalent Thickness (CET: a value obtained by reducing a dielectric capacity indicated by a material used for a gate insulator to a film thickness of a SiO2 film indicating the dielectric capacity equivalent to that of the material) can be suppressed to such a film thickness that no tunneling current It flows, as well as a physical film thickness can be increased. Therefore, generation of the tunneling current It in the gate insulating film is suppressed and as a result, the leakage current can be reduced.
However, when using a high dielectric constant material, the following problems occur.
Hereinafter, a description will be made with reference to FIGS. 11 and 12.
FIG. 11 is a schematic cross sectional view of a gate section.
The gate section in FIG. 11 has the following structure. That is, a high dielectric constant insulating film 330 is formed on an oxide film 320 formed on a semiconductor substrate (not shown). Further, an insulating film 340 having a silicon (Si) content larger than that of the high dielectric constant insulating film 330 is formed on the film 330. Further, a gate electrode 350 made of polysilicon is formed on the film 340.
FIG. 12 shows the gate voltage versus drain current characteristics of the MOSFET. In FIG. 12, there is shown the gate voltage versus drain current characteristics of a p-MOSFET and n-MOSFET having in the gate section the high dielectric constant insulating film 330 made of an oxynitride film or a high dielectric constant material. According to FIG. 12, when the high dielectric constant insulating film 330 is made of a high dielectric constant material, the threshold voltage shifts from 0.6 to 0.7 V particularly in the p-MOSFET, as compared with a case where the film 330 is made of an oxynitride film. As a result, the thresholds of the p-MOSFET and the n-MOSFET are different from each other to cause a problem that a CMOS circuit cannot be constituted.
As for the cause of the threshold shift, the following is considered. In a manufacturing process of the MOSFET, a heat treatment is performed after the impurity doping in order to form a source/drain region. On this occasion, a metal element 30 such as hafnium (Hf) in the high dielectric constant insulating film 330 is caused to diffuse by heat energy, as shown in FIG. 11. Near a boundary between the insulating film 340 and the gate electrode 350, the diffusing metal element 30 reacts and bonds with a Si element in the gate electrode 350 to form a dipole as well as to generate an electric field. Thus, the electric field generated near the boundary between the insulating film 340 and the gate electrode 350 causes the shift of the threshold. Consequently, when using a high dielectric constant material, operation of a CMOS circuit becomes impossible.